1. Field of the Invention
The present invention generally relates to a frequency synthesizer, and more particularly, to a wideband frequency synthesizer with a feedback injection locking mechanism.
2. Description of Related Art
Wireless communication products and techniques, such as mobile phones, wireless local area networks (WLAN), Bluetooth, ultra wide bands (UWB), industrial scientific and medical (ISM) radio bands, and worldwide interoperability for microwave access (WiMAX) have been widely used in our daily life along with the development of wireless technologies in recent years. Thus, frequency synthesizers have been developed and applied to meet the requirements of local oscillating signals and communication channel selection in wireless communication systems.
To be specific, a frequency synthesizer provides a local oscillating signal to a radio frequency transceiver for the purpose of the frequency up/down conversion and linear modulation. Accordingly, the quality of the local oscillating signal is a major factor in signal transmission performance. The spectrum purity of the local oscillating signal affects not only the accuracy of the transmitted data but also the quality of the system channel. Particularly, in a multi-carrier system (for example, an orthogonal frequency division modulation (OFDM) communication system), high-density orthogonal carriers require high spectrum purity of the local oscillating source. In a wireless and frequently-used application, a frequency synthesizer has to be able to generate a pure and stable local oscillating signal in order to be integrated with a wireless transceiver.
Additionally, with the fast spreading of applications of communication technologies, large quantities of data are desired to be transmitted quickly. As a result, a wide transmission frequency bandwidth is desired to increase the bit transmission rate. Because a conventional frequency synthesizer has only one voltage-controlled oscillator, the frequency synthesizer usually has a narrow operating bandwidth therefore cannot meet the requirement of an existing multi-standard communication system for operating bandwidth. Thus, many wideband frequency synthesizers have been developed and discussed in recent years. In such a conventional wideband frequency synthesizer, multiple phase-locked loop circuits or multiple frequency dividers are adopted along with a multiplexer to make the output frequency to fall on different operating frequency band. In addition, a combination of different frequency synthesizers also allows the output frequency to fall on different operating frequency band therefore can meet the requirement of a broadband application.
However, the conventional techniques usually require very complicated hardware circuit designs and a long time for locking the frequency. In other words, a conventional wideband frequency synthesizer usually comes with a large circuit structure, a high power consumption level, and accordingly a high manufacturing cost. The frequency synthesizer integrated circuit in an existing communication system should offer a high efficiency and a low cost. Thereby, how to design a frequency synthesizer which is ideal and satisfactory has become a major subject in the industry.